CHARACTERIZING DIAGENESIS AND ITS OPPOSING IMPACTS ON POROSITY IN THE WEBER AND MADISON FORMATIONS IN SOUTHWEST WYOMING: APPLYING GEOCHEMICAL, PETROGRAPHIC, AND ISOTOPIC ANALYSIS TO DESCRIBE RESERVOIR HETEROGENEITY IN POTENTIAL CO2 STORAGE SITES

This study – funded under U.S. Department of Energy National Energy Technology Laboratory award DE-FE0002142 – uses petrophysical, isotopic, geochemical, and petrographic data to characterize the nature and extent of diagenetic alteration and its impact on reservoir porosity and heterogeneity of two targeted CO₂storage formations. Analyses were performed on core obtained from the Madison Limestone and Weber Sandstone from a 12,810-foot University of Wyoming stratigraphic test well at the Wyoming Carbon Underground Storage Project (WY-CUSP) study area located on the Rock Springs Uplift in southwest Wyoming.

The Pennsylvanian Weber Sandstone is a quartz arenite, carbonate-bearing sandstone with near-shore and eolian facies. The uppermost section is predominantly eolian and has the highest porosity values (8-12%). Petrographic evidence implies several episodes of alteration: early calcite cementation, anhydrite crystallization, quartz dissolution and overgrowth, and late-stage dolomitization controlled by primary depositional features. Most diagenetic events contributed to a lower overall porosity.

The Mississippian Madison Limestone can be divided into three units: a micritic calcitic upper section, a highly-altered dolomitic middle zone, and a mixed primary and altered basal layer of dolomites and limes. The middle zone has the highest porosity values (>25%), and is the primary target for investigation. The dolomite of the middle zone exhibits both micro- and coarse crystallinity, often in close proximity. Evidence for further diagenesis includes stylolites, late-stage anhydrite and calcite infilling, brittle fractures, and dissolution, all of which obliterated primary textures. Isotopic ratios of oxygen (-5.5 to 2.4‰) and carbon (-4.8 to 0.8‰) from the dolomite suggest a low temperature origin. Furthermore, oxygen isotopic enrichment trends with depth and negatively correlates with some trace elements, indicating increased alteration with depth. This is further evidenced by relatively depleted isotopic ratios measured in the late-stage calcite veins.

Though diagenesis had opposing effects on porosity in both reservoirs, the eolian section of the Weber Sandstone and the middle zone of the Madison Limestone are the best potential intervals for CO₂ storage on the Rock Springs Uplift.